(1) Field of the Invention
The present invention relates to a reflecting mirror and more particularly to a multi-layered back reflecting mirror superior particularly in freedom from glare, visibility, decorative aspect, productivity, cost, etc. The multi-layered back reflecting mirror of the present invention is used mainly as a rearview mirror for automobiles, a mirror with a convex surface that warns drivers against hazards on the road ahead, an ornamental mirror, etc. and further has other wide applications.
(2) Description of the Prior Art
As an example of the multi-layered back reflecting mirror comprising a glass substrate, a dielectric multi-layered film formed at the back side of the glass substrate and a metal film formed on the dielectric multi-layered film, Japanese Patent Application Kokai (Laid-Open) No. 212704/1985 discloses a multi-layered back reflecting mirror wherein a dielectric multi-layered film is formed by laminating a high refractive index material layer having an optical thickness of .lambda./4 (.lambda. is a wavelength) and a low refractive index material layer having an optical thickness of .lambda./4 by turns and further laminating thereon a high refractive index material layer having an optical thickness of .lambda./2. Further, Japanese Patent Application Kokai (Laid-Open) No. 212705/1985 discloses a multi-layered back reflecting mirror wherein a dielectric multi-layered film is formed by laminating a high refractive index material layer having an optical thickness of .lambda./4 and a low refractive index material layer of an optical thickness of .lambda./4 by turns so that the number of the total layers becomes four. Furthermore, U.S. Pat. No. 4,673,248 discloses a multi-layered back reflecting mirror comprising a glass substrate, a low refractive index material layer and a high refractive index material layer arranged in this order.
Japanese Patent Application Kokai (Laid-Open) No. 74005/1988 discloses a multi-layered back reflecting mirror comprising a glass substrate, a dielectric multi-layered film formed at one side of the glass substrate and a light-absorbing film formed on the dielectric multi-layered film, wherein the dielectric multi-layered film is formed by laminating four to eight dielectric layers having alternately differing refractive indices and at least one of these dielectric layers has an optical thickness of .lambda./2.
These conventional multi-layered back reflecting mirrors comprising a glass substrate, a dielectric multi-layered film formed at the back side of the glass substrate and a metal film formed on the dielectric multi-layered film, have spectral reflection characteristics as shown in FIG. 2.
In FIG. 2, a curve (a) is the spectral reflection characteristic of the multi-layered back reflecting mirror disclosed in Japanese Patent Application Kokai (Laid-Open) No. 212704/1985. As is clear from the curve (a), in this reflecting mirror, the reflectance is high in a wavelength region from 430 nm to 550 nm and decreases significantly from 550 nm to 700 nm, and the reflected light is bluish. Accordingly, the color balance of the background view provided by the mirror is greatly different from the actual color balance and a red color, in particular, is difficult to recognize, thus impairing the visibility of the mirror.
FIG. 3 shows the spectral luminous efficiency V' (.lambda.) of the human eye [curve (1)], the spectral energy characteristic P (.lambda.) of an automobile halogen headlight [straight line (2)] and their product P (.lambda.).times.V'(.lambda.) [curve (3)]. As is clear from FIG. 3, the wavelength of a light emitted from an automobile headlight (halogen lamp) at night and sensible by the human eye lies mainly in a wavelength region of 480-550 nm. In the case of the reflecting mirror disclosed in Japanese Patent Application Kokai (Laid-Open) No. 212704/1985, the wavelength region (430-550 nm) giving a high reflectance in the curve (a) of FIG. 2 coincides fairly well with the above wavelength region of 480-550 nm; accordingly, the mirror's ability to prevent the glare of the headlights of an automobile running behind is not as high as expected.
Also in FIG. 2, a curve (b) is the spectral reflection characteristic of the multi-layered back reflecting mirror disclosed in Japanese Patent Application Kokai (Laid-Open) No. 212705/1985, and a curve (c) is the spectral reflection characteristic of the multi-layered back reflecting mirror disclosed in U.S. Pat. No. 4,673,248. These curves have about the similar pattern as the curve (a). Therefore, it is apparent that these reflecting mirrors have insufficient visibility and insufficient glare prevention, similarly to the aforementioned reflecting mirror disclosed in Japanese Patent Application Kokai (Laid-Open) No. 212704/1985.
Meanwhile, the conventional multi-layered back reflecting mirror disclosed in Japanese Patent Application Kokai (Laid-Open) No. 74005/1988 is superior in visibility and freedom from glare but has drawbacks in that the productivity is low incurring a higher cost because the number of layers of the dielectric multi-layered film is as many as 4-8 and the formation (coating and baking) of the light-absorbing film must be made in a step different from the vacuum deposition step for formation of the dielectric multi-layered film.